Graphene 1-5 has been under intensive attention since its discovery in 2004 because of its unique chemical and physical properties and its importance in technological applications. More recently, layered semiconductors with finite band gaps 6-11 have emerged as unique platforms for studying fundamental surface chemistry and as promising candidate materials for the next-generation nano-electronics, which are complementary to graphene-based materials with zero or very small band gap. Of particular interest are metal dichalcogenides and halides stable in the form of nano-membranes with a thickness down to single or a few atomic/molecular layers, which not only provide unique platforms for studying fundamental sciences, but also possess significant potentials for a wide scope of applications such as transistors, memory devices and energy storage. For example, significant attention has been directed to molybdenum disulfide (MoS2) 8-10, a metal dichalcogenide with an indirect band gap of 1.29 eV in the bulk form but, interestingly, undergoing a transition toward a direct band gap of 1.9 eV in the form of a single atomic-layer.9,10 
Recently, it has been shown that exfoliated crystalline nano-membranes of SnS2, another layered semiconductor with a band gap of 2.1 eV, show great potential as the building blocks for nano-electronics.12 Bulk SnS2 is a layered material (FIG. 1) with a hexagonal CdI2 type crystal structure. The crystals consist of stacked S—Sn—S trilayers which are bonded by strong covalent forces within each trilayer but only weakly coupled to adjacent trilayers via van der Waals interactions. As a consequence of the weak inter-layer force, it is possible to realize stable thin nano-membranes of such materials with a thickness down to single or a few atomic/molecular layers.
However, despite its technological importance, the synthesis of thin crystal arrays of such 2D semiconductors at designed locations on suitable substrates has not been realized. The creation of such single and few atomic layers (membranes) is usually achieved by using mechanical and chemical exfoliation techniques to separate bulk crystals into multi-layers (FIG. 1). For example, a widely adopted technique is to use an adhesive Scotch tape for mechanical exfoliation, with little control on the yield and the thickness of the resultant membranes. Subsequently, by complicated solution-based processing, such exfoliated membranes are transferred on to a suitable substrate (e.g. SiO2/Si) for the purpose of characterization and device fabrication, which further limits the usefulness of the exfoliation approach in technological applications of such semiconductor membranes. In particular, most practical applications require the controlled placement of arrays of such thin semiconductor crystals on the designed locations on suitable substrates. To our knowledge, there has been no report of the direct synthesis of such crystalline thin semiconductor arrays on desired locations on suitable substrates, which is critical for many practical applications. The present disclosure provides a novel approach to the controlled synthesis of thin crystal arrays of SnS2 and SnS at predefined locations on chip, by integrating a top-down process—standard nanofabrication, and a bottom-up process—chemical vapor deposition. This integrated process provides single- or a-few-atomic/molecular-layer thick or micron-thick thin crystal grains of layered semiconductors directly on suitable substrates at predefined locations, which is suitable for wafer-scale production and compatible with the strategy for integrated circuits in semiconductor industry. The present disclosure provides novel chemical routes to atomic-layer-thick semiconductor crystal grains, and lays down the foundation for the future integration of such low-dimensional nano-materials in industrial applications. This opens a pathway for the large-scale production of layered 2D semiconductor devices and lays down the foundation for their future applications in integrated nano-electronic/photonic systems.